The invention relates to a progressing cavity fluid motor for directly driving a drill bit. Motors of this kind based on the Moineau principle find application to a considerable extent in deep drilling as direct drive bits or so-called "bottom motors". In these situations they are provided with an upper junction end on the housing for a connection with the drilling pipe. The motor drives the drill bit or similar drilling tool by way of a flexible shaft connecting the motor with the drilling tool. In this type of motor the flushing medium (drilling mud) is pumped downwards under high pressure into the progressing cavity work space in the motor between a stator forming the housing and the rotor forming the shaft. On its helical path through the motor a part of the pressure energy of the drilling mud is converted into rotational energy for the shaft. The pressure drop inside such motors, depending on the constructional design and direct drive drilling carried out in practice, is in the order of 25 or 60 bars.
With the well-known fluid motors the stator is in the form of a helical female thread and has an inner lining of an elastic deformable material secured to the housing. Inner linings of this kind are expensive to manufacture. For a satisfactory operation of the motor it is important that the molded helical thread surface in the working space be in contact with sufficient amount of the male helical rotor surface to seal against leakage. However excess pressure between the molded stator surface and the rotor results in increased wear of the molded surface and the performance of the motor drops; the design value of the motor will not be achieved. The determining contact pressure for the molded surfaces for an acceptable sealing in the contact areas of such fluid motors is usually determined so as to be in excess of a minimum; in so doing the pressure of the flushing medium is taken into account in the work space of the motor which tends to separate the motor surfaces from one another. In addition to pressure one must consider the temperature conditions under which a motor has to operate. This means that for the achievement of optimum operating conditions for the motor the latter must be adjusted for the existing operating conditions in the drill hole within narrow limits. This requires not only an expensive multiplicity of motors but also a most exact prognostication or predetermination of the drilling operating conditions in order to be able to prepare a suitable motor construction design. If the actual operating conditions deviate from the former which were established for the motor design either a loss of efficiency or increase in wear will occur.
With the aid of differential load measurements the contact pressures for the molded surfaces can be determined only within a limited extent with the result that with the help of such motors the torque capable of being generated is limited. While one can get by with relatively low torques with a single threaded helical motor chamber. With larger torque motors the helical shaped surfaces have a kind of a multiple thread interlacing gearing, that is the shafts for example have nine helical gears and the housings have ten screw threads. Such multiple designs however are often not sufficient to produce the necessary torque, in which cases integral drive parts are introduced in which several motors are connected in series coaxially. Integral drive parts of this kind, however, are not only extraordinarily large but also extraordinarily expensive and indeed not only in manufacturing but also in maintenance.